Alkyl transfer of alkyl aromatics with croup vi-b metals on type y zeolites

ABSTRACT

A PROCESS FOR THE ALKYL TRANSFER OF ALKYL AROMATICS INCLUDING CONTACTING AN ALKYL AROMATIC FEED MATERIAL, SUCH AS TOLUENE, WITH A CATALYST COMPRISING A GROUP VI-B METAL, SUCH AS CHROMIUM, MOLYBDENUM, TUNGSTEN, DEPOSITED ON A TYPE Y ZEOLITE BASE AT A TEMPERATURE OF ABOUT 700 TO 1100*F., A PRESSURE OF ABOUT 0 TO 2000 P.S.I.G., AND A LIQUID HOURLY SPACE VELOCITY OF ABOUT 0.1 TO 10, AND IN THE PRESENCE OF HYDROGEN INTRODUCED AT A RATE OF ABOUT 1 TO 10 MOLES HYDROGEN PER MOLE OF HYDROCARBON FEED. PROMOTERS SELECTED FROM GROUP I, GROUP II, GROUP IV, AND THE RARE EARTH METALS OF THE PERIODIC SYSTEM MAY BE ADDED TO THE CATALYST. DEACTIVATED CATALYST MAY BE PERIODICALLY REJUVENATED BY DISCONTINUING THE INTRODUCTION OF AROMATIC FEED MATERIAL AND PURGING WITH HYDROGEN AND THE CATALYST CAN BE REACTIVATED BY CALCINATION IN AN ATMOSPHERE SUCH AS AIR. WHERE TOLUENE IS THE FEED, THE ALKYL TRANSFER PRODUCT MAY BE DISTILLED TO SEPERATE BENZENE, TOLUENE AND XYLENES, THE TOLUENE MAY BE RECYCLED TO THE ALKYL TRANSFER STEP, THE XYLENES MAY BE CRYSTALLIZED TO SEPARATE PARA-XYLENE FROM THE REMAINING XYLENES, THE MOTHER LIQUOR FROM THE CRYSTALLIZATION STEP MAY THEREAFTER BY ISOMERIZED TO READJUST THE PARA-XYLENE CONTENT AND THE PRODUCT OF THE ISOMERIZATION MAY BE RECYCLED TO THE CRYSTALLIZATION ZONE.

3,597,491 S. M. KOVACH ErL ALKYL TRANSFER oF ALKYL Anonmrcs wrm GROUPvI-B Aug. 3, 1971 METALS 0N TYPE Y ZEOLITES Filed D66. 19, 1968 ATTORNEYUnited States Paten U.S. Cl. 260-672R 9 Claims ABSTRACT OF THEDISCLOSURE A process `for the alkyl transfer of alkyl aromaticsincluding contacting an alkyl aromatic feed material, such as toluene,with a catalyst comprising a Group VI-B metal, such as chromium,molybdenum, tungsten, deposited on a Type Y zeolite base at atemperature of about 700 to 1100 F., a pressure of about O to 2000p.s.i.g., and a liquid hourly space velocity of about 0.1 to 10, and inthe presence of hydrogen introduced at a rate of about l to 10 moleshydrogen per mole of hydrocarbon feed. Promoters selected from Group I,Group Il, Group IV, and the Rare Earth metals of the Periodic System maybe added to the catalyst. Deactivated catalyst may be periodicallyrejuvenated by discontinuing the introduction of aromatic feed materialand purging with hydrogen and the catalyst can be reactivated bycalcination in an atmosphere such as air. Where toluene is the feed, thealkyl transfer product may be distilled to separate benzene, toluene andxylenes, the toluene may be recycled to the alkyl 4transfer step, thexylenes may be crystallized to separate para-xylene from the remainingXylenes, the mother liquor from the crystallization step may thereafterbe isomerized to readjust the para-xylene content and the product of theisomerization may be recycled to the crystallization zone.

BACKGROUND OF THE INVENTION The present invention relates to a processfor the catalytic conversion of hydrocarbons and, more particularly, toa process for the catalytic alkyl transfer of alkyl aromatics.

Aromatic hydrocarbons, such as benzene, naphthalene, and their alkylderivatives are important building blocks in the chemical andpetrochemical industries. For example, benzene and its derivatives havenumerous uses; cyclohexane is utilized in nylon production; naphthaleneis utilized in the production of phthalic anhydride for alkyd resins,etc.; para-xylene can be used for the production of terephthalic acidwhich, in turn, is utilized in the production of synthetic resins, suchas Dacron, Mylar, etc., etc.

For many years, the primary source of such aromatic hydrocarbons hasbeen `coal tar oils obtained by the pyrolysis of coal to produce coke.Such coal tar oils con- Itain principally benzene, toluene, naphthalene,methylnaphthalene and para-xylene. Benzene maybe produced from such oilsby direct separation, such as distillation techniques, the para-xylenemay be separated by crystallization, and the naphthalene fractions bydirect separation techniques. Further alkyl derivatives of benzene andnaphthalene can be converted to increased volumes of benzene andnaphthalene by hydrodealkylation.

More recently, however, the petroleum industry has become a leadingsource of these aromatic hydrocarbons. The reason for this has been theavailability of the catalytic reforming process in which naphthenehydrocarbons are dehydrogenated to produce a reformate rich in aromaticsand more efficient processes for separating the aromatics from thereformate.

Some years ago, there was a high demand for toluene which was used inthe production of TNT. This led to the building of substantialfacilities for its production. However, the advent of nuclear and fusionweaponry and the use of diesel oil-ammonium nitrate explosives has lefttoluene in substantial over-supply, since the only major uses of tolueneare as a solvent, the production of toluene diisocyanates and theproduction of benzene. This has resulted in extensive efforts to developmethods for converting toluene to benzene. One method of convertingtoluene to benzene is by the previously mentioned hydrodealkylation.

Dealkylation has the primary disadvantage that methane is a majorproduct. Volume yields of benzene are therefore low and carbondeposition on the catalyst is high. The large amounts of methane, whileuseful as a fuel, require expensive techniques forl the removal of theImethane from the circulating hydrogen stream utilized in thehydrodealkylation. In addition, large quantities of hydrogen areconsumed in the dealkylation process and hydrogen is often in shortsupply and expensive to produce. Finally, where catalysts are used inthe process, carbon laydown on the catalyst is a serious problem.

A more profitable reaction for changing alkyl aromatics to otheraromatic products is an alkyl transfer reaction. An alkyl transferreaction is a process wherein alkyl groups are caused to be transferredfrom the nuclear carbon atoms of one aromatic molecule to the nuclearcarbon atoms of another aromatic molecule. By way of example, anaromatic hydrocarbon molecule containing one nuclear alkyl substituent,such as toluene, may be treated by disproportionation to produce anaromatic hydrocarbon with no alkyl substituents, namely, benzene, andaromatic hydrocarbon molecules with two nuclear alkyl substituents,namely Xylenes. Similarly, product ratios may be shifted bytrausalkylation of xylene and benzene to toluene. Such an alkyl transferreaction has distinct advantages: methane is not produced, but instead,valuable aromatic hydrocarbons are produced in addition to the desiredaromatic hydrocarbon. As a result, there is very little loss of productin alkyl transfer as opposed to hydrodealkylation.

Alkyl transfer may be carried out thermally. However, thermal alkyltransfer results in demethylation due t0 cracking and hydrogenation,ultimately resulting in low yields of desired aromatics. On the otherhand, catalytic alkyl transfer has not been highly successful since itrequires an active, rugged, acidic catalyst. Typical catalysts are solidoxides, such as silica-alumina, silica, magnesium, etc. These materials,however, are not active enough to promote disproportionation at highconversion rates. In addition, as is the case in hydrodealkylation,carbon deposition on the catalyst and its affect o ncatalyst activityWith time is a severe problem.

It is therefore an object of the present invention to provide animproved process for the conversion of alkyl aromatics. Another objectof the present invention is to provide an improved process for the alkyltransfer of alkyl aromatics. Yet another object of the present inventionis to provide an improved process for the disproportionation of tolueneto produce benzene and Xylenes. Another and further object of thepresent invention is to provide an improved process for thedisproportionation of alkyl aromatics which utilizes a novel catalystsystem. Another object of the present invention is to provide animproved process for the disproportionation of alkyl aromatics with acatalyst system resistant to carbon laydown. A further object of thepresent invention is to provide an improved process for the catalyticdisproportionation of alkyl aromatics utilizing a Group VIII metal andboria on an alumina base. A further object of the present invention isto provide an improved process for the disproportionation of alkylaromatics utilizing critical conditions of temperature and pressurewhich produce maximum disproportionation and conversion of one aromaticto another. Still another object of the present invention is to providean improved process for the conversion of toluene to benzene and xylenesand conversion of metaand ortho-xylenes to additional para-xylene. Theseand other objects and 'advantages of the present invention will beapparent from the following detailed description.

SUMMARY OF THE INVENTION Briefly, in accordance with the presentinvention, alkyl transfer of alkyl aromatics comprises contacting analkyl aromatic feed material with a catalyst comprising a metal of GroupVI-B of the Periodic System deposited on a Type Y zeolite base. Furtherimprovement of the catalyst is obtained by adding a Group I, Group II,Group IV, a Rare Earth metal or mixtures thereof. Further improvementsof the process are obtained by maintaining the temperature between about700 and ll F. and the pressure between about 0 and 2000 p.s.i.g. Wheretoluene is the feed, additional para-xylene is produced by isomerizingorthoand meta-xylenes.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing shows a flow diagram of aprocess system in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Alkyl aromatic feed materials foruse in accordance Wtih the present invention can be any alkyl aromatichaving a least one transferrable alkyl group. Primary materials .arealkyl aromatics having from 7 to 15 carbon atoms, mixtures of such alkylaromatic hydrocarbons, or hydrocarbon fractions rich in such alkylaromatic hydrocarbons. Such feeds include monoand di-aromatics, such asalkyl benzenes and alkyl naphthalenes. Preferably, the alkyl groupshould contain no more than about 4 carbon atoms. A preferred feed inaccordance with the present invention is toluene. Accordingly,disproportionatoin of toluene will be referred to hereinafter in thedetailed description.

The process of the present invention should be conducted at atemperature between about 700 and 1100 F., and p referably beween 800and 1000 F. It has been found in accordance with the present inventionthat below this temperature range, substantially decreased conversionoccurs due to hydrogenation. On the other hand, when operating abovethis temperature range, thermal demethylation occurs. The pressureutilized in accordance With the present invention has also beendetermined to be a critical factor. Accordingly, the process should becarried out between about 0 and 2000 p.s.i.g., and preferably between300 to 600 p.s.i.g. It has been found that below the `desired pressurerange, conversion is low and the aromaticity of the product is high. Onthe other hand, at higher pressures, conversion is high, but liquidrecoveries are low due to hydrogenation and hydrocracking. A liquid`hourly space velocity between about 0.1 and l0, and preferably between0.25 and 1.0, should be utilized and a hydrogento-hydrocarbon mole ratiobetween about 1 and 10 to 1 and preferably between l and 2 to l isdesired.

The high severity conditions required to obtain disproportionation ofalkyl aromatics, particularly the disproportionation of toluene, hasbeen found to lead to catalyst deactivation due to selective adsorptionand condensation of aromatics on the catalyst surface and carbon laydownon the catalyst. It was found that the condensation and adsorption ofaromatics on the catalyst is a temporary poison and that this conditioncan be alleviated by utilizing high hydrogen partial pressures. Inaddition, this ternporary deactivation of the catalyst can be overcometo completely rejuvenate the catalyst to near virgin activity byhydrogen-purging of the catalyst in the absence of aromatic hydrocarbonfeed. While coke or carbon deposition on the catalyst is a permanentpoison, it has been found, in accordance with the present invention,that carbon laydown can be decreased by utilizing the catalysts of thepresent invention. Further, it was found that when these catalystsbecome deactivated by carbon laydown, they can be restored to nearvirgin acivity by regeneration in air.

The zeolites are a class of hydrated silicates of aluminum and eithersodium or calcium or both, having the general fromulaNazO-AlzOg'nSiOzxHzO. Originally, the term zeolite described a group ofnaturally-occurring minerals which were principally sodium or calciumaluminosilicates. Such naturally-occurring zeolites include, forexample, chabazite, gmelinite, erionite, faujasite, analcite,heulandite, natrolite, stilbite, thomsonite, etc. Synthetic zeolites aregenerally known in the trade by a trade name or trade designationapplied by the specic manufacturer. For example, Types A, X, and Y aremanufactured by Linde Company. Zeolites generally have a rigid,threedimensional anionic network with intracrystalline channels whosenarrowest cross-section has essentially a uniform diameter. Thus,zeolites, of both natural and synthetic origin, can be distinguishedfrom crystalline aluminosilicate clays, such as bentonite, which have atwo-dimensional layer structure, and silica-alumina synthetic catalystswhich are amorphous aluminosilicates having a random structure.

Zeolites whose atoms are arranged in a crystal lattice in such a waythat there are a large number of small cavities innerconnected bysmaller openings or pores of precisely uniform size are generallyreferred to as molecular sieves. Some natural zeolites exhibit molecularsieve characteristics to a limited degree. However, the syntheticzeolites as a class exhibit these characteristics.

The synthetic zeolite found particularly effective in the presentprocess is a Type Y molecular sieve manufactured by Linde Company. Thismaterial has the general formula Na56+y[(AlO2)56+y(SiO2)136 y] where yhas a value of about 0 and can vary from -8 to 20. The Type Y zeolitecrystallizes in the cubic system and the lattice constant for the sodiumform, with a Si/Al ratio of 2.5, is 24.66 A. In the sodium form of Y,the negative charge in each A104 is balanced by a closely associatedsodium atom. In the divalent cationic form, however, the divalentcation, usually calcium or magnesium, is asymmetrically located withrespect to the aluminas. The calcium form is preferred in thisinvention.

Processes for depositing the active metal of the present invention onthe zeolites are Well known in the art. This simply consists ofreplacing all or a part of the exchangeable cations of the zeolite withthe metallic ions by ion exchange.

The Type Y may also be utilized in accordance with the present inventionby rst converting the sodium or calcium form to the hydrogen form whichis often referred to as the acid form. Conversion to the hydrogen formmay be carried out by either replacement of the metal Ycations withhydrogen ions or by replacement of the metal ions with ammonium ions,followed by the composition of the ammonium by calcination. By thistechnique, from to 99% of the metal is removed and replaced by hydrogen.Thereafter, the desired active metal is exchanged for the hydrogen toform the finished catalytic material.

The active Group VI-B metal includes chromium, molybdenum and tungsten,preferably in their oxide form. The Group VI metal should be present onthe finished catalyst in amounts between about 1.0 and 20% by weight.

It has also been found that conversion may be mproved and, moresignificantly, carbon laydown on the catalyst may be reduced by theaddition thereto of a promoter. Sulch promoters may be selected fromGroup I of the Periodic System, such as potassium, rubidium, cesium,etc., Group Il of the Periodic System such as calcium, magnesium,strontium, etc., a Rare Earth metal of the Periodic System, such ascerium, thorium, etc., a Group IV metal of the Periodic System, such astin or lead, for mixtures of these, and particularly mixtures of a GroupIV metal with one of the other groups mentioned. The promoters arepreferably in their oxide form and are present in amounts of about 1 to15% by weight based on the weight of the finished catalyst.

The following table compares a Jchromia on Type Y catalyst with otherdisproportionation catalysts, utilizing toluene as a feed.

e Type Y sieve.

A second important advantage of the catalyst of this invention (run 3)as compared to those in runs `l and 2 is catalyst activity in relationto the hydrogen ratio. The catalysts in run 1 and 2 require a highhydrogen ratio (at least 3 to 1) to maintain catalyst activity over areasonable length of time. The catalyst of this invention only requiresa low hydrogen to hydrocarbon ratio (1-2/ 1 ratio). Employing thisratio, the process has been operated for several days without anydecline in catalyst activity as opposed to an 8-24 hour cycle for theother catalyst system.

The catalyst of this invention has excellent regeneration properties. Itwas regenerated in air without loss of activity. This is shown in TableII.

TABLE II.-CHROMIA ON TYPE Y SIEVE In accordance with the presentinvention, an integrated process for the production of benzene andpara-Xylene from toluene can be carried out with resultant high yieldsof these two valuable products. This process is best described byreference to the drawing.

In accordance with the drawing, toluene is introduced to the systemthrough line 10, hydrogen is added through line 12 and these materialsare passed over the catalyst of the present invention in thedisproportionation reactor 14. The effluent passing through line 16 ispassed to a flash drum for the removal of hydrogen and any light gasesproduced. These materials are discharged through line 18. Since littleor no demethanation occurs, the hydrogen is substantially pure and maybe recycled to the disproportionation reaction without furthertreatment.

However, in some instances, further purication of the hydrogen isnecessary before recycle or reuse. The liquid product passes throughline 20 to a first distillation unit 22. In distillation unit 22,benzene is recovered as an overhead through line 24. The bottoms productfrom distillation unit 22 passes through line 26 to a seconddistillation unit 28. In distillation unit 28, toluene is removed as anoverhead product and recycled to the disproportionation section throughline 30. The bottoms product from distillation unit 28 is a mixture ofxylenes which is discharged through line 32. This product may bewithdrawn, as such, through line 34. Preferably, however, the Xyleneproduct is passed through line 36 to crystallization unit 38. Incrystallization unit 38, para-Xylene is selectively removed andwithdrawn through line 40. The mother liquor from the crystallizationsection is passed through line 42 to an isomerization unit 44. Hydrogenis added through line 46. In the isomerization unit 44, the equilibriumconcentration of para-xylene is re-established and the material may thenbe recycled through line 48 to crystallization unit 38 for furtherpara-Xylene separation.

The isomerization reaction should be carried out under more mildconditions than the disproportionation. Catalysts useful in thedisproportionation reaction might also be used in the isomerization orconventional catalysts, such as, platinum on silica-alumina, can beused. The isomerization may be carried out at temperatures of about 500to 900 F., and preferably 550 to 650 F., pressures of 50 to 2000p.s.i.g., and preferably 300 to 600 p.s.i.g., at a liquid hourly spacevelocity of 0.1 to 10, and utilizing a hydrogen-to-hydrocarbon moleratio between about l and 20 to 1.

When reference is made herein to the Periodic System of Elements, theparticular groupings referred to are as set forth in the Periodic Chartof the Elements in The Merck Index, Seventh Edition, Merck & Co., Inc.,1960.

The term alkyl transfer of alkyl aromatics as used herein is meant toinclude disproportionation and transalkylation. Disproportionation, inturn, is meant to include conversion of two moles of a single aromatic,such as toluene, to one mole each of two different aromatics, such asXylenes and benzene. Transalkylation is meant to include conversion ofone mole each of two different aromatics, such as Xylenes and benzene,to one mole of a single different aromatic such as toluene. The alkyltransfer defined above is also to be distinguished from isomerizationwhere there is no transfer of alkyl groups from one molecule to anotherbut simply a shifting of alkyl group around the aromatic ring, such asisomerization of xylenes, or rupture of the ring or the alkyl side chainand rearrangement of split-off carbon atoms on the same molecule. Thealkyl transfer is also to be distinguished from a hydrogen transferreaction, such as the hydrogenation of aromatics, the dehydrogenation ofcycloparaiiins and like reactions.

What is claimed is:

1. A process for the alkyl transfer of alkyl aromatics; comprising,contacting an alkyl aromatic feed material with a catalyst comprising 1to 20% by weight of a Group VI-B metal of the Periodic System depositedon a Type Y zeolite base, at a temperature of about 700 to l F., apressure of about 0 to 2000 p.s.i.g., a liquid hourly space velocity ofabout 0.1 to 10, and a hydrogento-hydrocarbon mol ratio between about 1and 10 to 1 to cause transfer of alkyl groups of said aromatics.

2. A process in accordance with claim 1 wherein the Group VI-B metal ischromium.

3. A process in accordance with claim 1l wherein the feed materialcontains substantial volumes of toluene.

4. A process in accordance with claim 3 wherein unconverted toluene isseparated from the alkyl transfer product and said unconverted tolueneis recycled to the disproportionation step.

5. A process in accordance with claim 3 wherein Xylenes are separatedfrom the alkyl transfer product and paraxylene is separated from saidxylenes.

6. A process in accordance with claim 5 wherein the Xylenes remainingafter the separation of para-Xylene are subjected to isomerizationconditions sufficient to produce additional para-Xylene.

7. A process in accordance with claim 1 wherein the flow of feedmaterial through the catalyst is interrupted periodically and the flowof hydrogen is continued for a time sufcient to reactivate the catalyst.

8. A process in accordance with claim 1 wherein the flow of feedmaterial and hydrogen through the catalyst is discontinued and thecatalyst is calcined in air under conditions sufficient to reactivatethe catalyst.

9. A process in accordance with claim 1 wherein the catalystadditionally contains about 1 t0 15% by weight of a .metal selected fromthe group consisting of Group I,

8 Group II, Group IV and the Rare Earth metals of the Periodic Systemand mixtures thereof.

References Cited DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS,Assistant Examiner

